Alkaline-earth oxides have long been used as electron emissive coatings on cathode surfaces, since they are known to be the most efficient materials for stable thermionic emission (i.e., they have the highest emission density for a given operating temperature). The limitations to using these materials in their pure form in very high emission density applications are:
(a) The resistivity at operating temperature is such that excessive Joule heating of the coating occurs, causing catastrophic breakdown effects in the coating (arcing and sparking).
(b) Pure barium oxide (BaO) (which is the most commonly used of these compounds) is an n type semiconductor, so that the work function is dependent on donor density. The donor density can be changed significantly by the absorption of small amounts of residual tube gases. This results in a cathode that is unacceptably sensitive to the poisoning effect of a high power tube environment, compared to alternative cathode structures.
(c) Pure BaO is usually applied to the surface of the cathode in the form of a sprayed on coating. The adherence of these coatings is such that mechanical shocks will often cause them to peel or flake off.
Various fabrication methods have been devised to try to overcome the limitations described above, and yet take advantage of the low work function properties of the alkaline earth oxide materials. Some of these are the following:
(a) Bariated Nickel-or Nickel Matrix Cathode--This cathode utilizes a pressed and sintered matrix of nickel which is formed from nickel powder. The matrix is then impregnated by various means with carbonates of the alkaline earth oxides. The carbonates are converted to oxides upon heating. The exposed oxide on the emitting surface provides low work function on that fraction of the surface which it occupies. The limitations of this structure are as follows:
(1) Because of the large pore sizes in the sintered nickel matrix (5-10 microns) the resistivity effects in the oxide particles are not completely eliminated. In addition, the fraction of the surface covered by oxide is approximately equal to the pore volume in the sintered matrix. This cannot be much greater than 30 percent and still maintain the mechanical integrity of the matrix. Thus the "good" emitting area of the surface is limited by that fraction.
(2) The size of the sintered plug must also be thick enough to maintain mechanical integrity. This usually requires a minimum thickness of 0.040 inches (depending on the cathode area). Thus, a lower limit on the mass of the cathode is determined by this factor. This will limit the rate of temperature rise of the cathode, and its consequent usefulness in "fast turn on" applications.
(3) The conventional technology is to use nickel for the matrix of these cathodes. Nickel is used primarily because it does not have a strong reducing interaction with the oxide. The technology of making matrices from more refractory metals that are also non-reducing has not been developed. The relatively high vapor pressure of nickel limits its operating temperature to less than 1000 degrees centigrade. This places an upper limit of less than 20 amperes/sq cm on the emission from these cathode structures.
(b) Coated Particle Cathode--Another approach to eliminating the resistivity problems in oxide cathodes is to coat small particles of oxide (500 to 1000 Angstroms Dia.) with nickel, and deposit a coating consisting of these particles on a nickel base. The major limitation of this cathode is, again, the low operating temperature imposed by the nickel. No technology is available to produce these cathode structures with metals other than nickel. In practice it is found that these cathodes are limited to a few amperes/sq cm for reliable performance.
(c) Dispenser Type Cathodes--Most cathodes that are used in high emission density applications are of this type. For these cathodes the operating mechanism is different from oxide types. Instead of the emission originating directly from an oxide surface, it comes from a surface onto which is diffused a monolayer of barium, which lowers the surface work function. The work function achieved in this way is considerably higher than that obtained on a well activated barium oxide surface (2.0 eV. compared to 1.5 to 1.7 eV. for the oxide). Thus, the operating temperature required to obtain emission levels greater than 20 Amperes/sq. cm. is greater than 1150 degrees Centigrade. This gives rise to excessive evaporation products from the cathode, and consequent reliability problems.